Question

I have a basic class ‘HistoryItem’ like so:

public class HistoryItem
  private Date startDate;
  private Date endDate;
  private Info info;
  private String details;

  @Override
  public int hashCode() {
    int hash = (startDate == null ? 0 : startDate.hashCode());
    hash = hash * 31 + (endDate == null ? 0 : endDate.hashCode());
    return hash;
  }
}

I am currently using a HashSet to remove duplicates from an ArrayList on the startDate & endDate fields, which is working correctly.

However I also need to remove duplicates on different fields (info & details).

My question is this.
Is there a way to specify a different method which HashSet will use in place of hashCode()?
Something like this:

public int hashCode_2() {
  int hash = (info == null ? 0 : info.hashCode());
  hash = hash * 31 + (details == null ? 0 : details.hashCode());
  return hash;
}

Set<HistoryItem> removeDups = new HashSet<HistoryItem>();
removeDups.setHashMethod(hashCode_2);

Or is there another way that I should be doing this?

Answer 1

I ended up using GNU Trove for this.

Minimal code change was required.

A new class implementing TObjectHashingStrategy (containing HashCode and Equals methods).

public class HistoryItemDuplicateInfo
implements TObjectHashingStrategy<HistoryItem> {

  @Override
  public int computeHashCode(HistoryItem obj) {
     ...
  }

  @Override
  public boolean equals(HistoryItem arg0, HistoryItem arg1) {
    ...
  }
}

Then use the THashSet object with a specified strategy for removing the duplicates.

THashSet<HistoryItem> hs = new THashSet<HistoryItem>(new HistoryItemDuplicateInfo());

Hope this is able to help someone out in future.